The present invention relates to brake shoe clearance adjusting device in internal shoe drum brakes.
One of conventional internal shoe drum brakes comprises, as shown, for example, in British Pat. No. 1199191 and No. 1274844, a pair of arcuate brake shoes movably mounted on a stationary brake structure, power actuated service brake means for expanding the shoes against the drum, auxiliary, mechanical brake actuating means including an actuating lever engaging with one shoe and a strut for transmitting auxiliary brake actuating force to the other shoe. The strut includes a first strut part having a tubular portion, a second strut part having male screw-threaded portion at least a portion of which being inserted into the tubular portion of the first strut part displaceably, and an adjusting nut engaging with the male screw-threaded portion of the second strut part and abutting with the open end of the tubular portion of the first strut part and having, usually, ratchet teeth formed on its outer periphery. The length of the strut is adjusted by rotating the adjusting nut on the second strut part to compensate the wear of brake shoes. The abutment of the adjusting nut with the first strut part is, usually, maintained by a shoe return spring extending between the brake shoes.
In the shoe drum brake of the aforementioned type, there is shortcomings such that, when it is intended to effect the adjustment in releasing the mechanical brake actuating means the biassing force of the return spring will not sometimes act on the strut under the reasons that the brake shoes will not return quickly to the retracted position by such as the resistance of hydraulic pistons in power actuated service brake means or the like, thereby the adjusting nut will separate from the open end of the tubular portion of the first strut part, and the rotational resistance of the adjusting nut on the second strut part will decrease substantially, as the results, the rotational direction of the adjusting nut cannot be controlled unitary and the adjustment of the shoe clearance cannot be effected reliably since the adjusting nut may sometimes be rotated in the reverse direction (in the direction contracting the effective length of the strut).
Further, when the biassing force of the shoe return spring does not act on the strut comprising a first and second strut parts, the position of the strut in the shoe drum brake cannot be maintained properly thereby causing rattling noise and undesirable wear of related parts.